Advanced semiconductor devices fabricated with passivated high aluminum content III-V materials

ABSTRACT

For AlGaAs LEDs the confining layers adjoining the active layer possess the highest Al composition. From failure analysis of non-passivated, WHTOL-aged, AlGaAs LEDs, it was discovered that corrosion occurs the fastest at the exposed surfaces of the high Al-content confining layers. By placing a high-quality native oxide at the exposed surfaces of the high Al-content confining layers which protect from the formation of the `poor` oxide, it is possible for LEDs to retain essentially their same light output after 2,000 hours of WHTOL testing. Further, it is possible to improve carrier confinement, carrier injection, wave guiding, and other properties by increasing the Al-content of different layers.

CROSS REFERENCE TO RELATED APPLICATION(S)

This is a divisional of copending application Ser. No. 09/231,411 filedon Jan. 14, 1999.

TECHNICAL FIELD

The present invention relates generally to the field of III-Vsemiconductor devices and more particularly to the fabrication ofdevices with aluminum (Al)-containing III-V semiconductor materials.

BACKGROUND ART

The general background of the present invention is disclosed in U.S.Pat. No. 5,262,360 by Holonyak, Jr. and Dallesasse (Holonyak '360)entitled "AlGaAs Native Oxide" granted Nov. 16, 1993 and U.S. Pat. No.5,517,039 by Holonyak, Jr. et al (Holonyak '039) entitled "SemiconductorDevices Fabricated with Passivated High Aluminum Content III-VMaterials" granted May 14, 1996. Holonyak '039 is co-assigned with thepresent invention to the Hewlett-Packard Company.

High aluminum (Al) containing III-V semiconductor materials degrade inwet; high-temperature environments due to the formation of anundesirable Al-oxide, primarily thought to be Al[OH]₃. These oxides tendto be optically absorbing and limit the transmission of light from lightemitting semiconductor devices. The poor Al-oxide can also attack thecrystal structure of the device.

One method of preventing device degradation is to grow a high qualitynative oxide that effectively seals the device and prevents theformation of a poor, optically absorbing Al-oxide. Native oxides areformed at higher temperatures and include Al(O)OH and Al₂ O₃. A deviceis considered passivated if the native oxide prevents or significantlyreduces the formation of the poor oxide (e.g., Al[OH]₃) when the deviceis operated in wet, high-temperature environments. Within thisdescription, there can be different degrees of passivation for devicesthat have been subjected to wet, high-temperature operating life (WHTOL)testing based on the amount of device degradation after operating for afixed time.

For example, a light emitting diode (LED) may be consideredfully-passivated if the emitted light output power (LOP) has degradedless than 20% after 2000 hours of WHTOL operation.

An LED is considered partially-passivated if the LOP has degraded lessthan 50% after 2000 hours of WHTOL operation. Thus, the term"passivated" describes devices from partially to fully passivated.Herein, the conditions of a WHTOL test are under 20 mA loading (i.e.,forward bias in an LED) in an atmosphere of 85% relative humidity and atemperature of 85° C.

A method for forming high quality native oxides through the use of awater vapor environment at elevated temperatures is described inHolonyak '360 and is applicable herein. A wide range of temperatures isdescribed between 375° C. to 1000° C. to grow native oxides inAl-bearing materials. In Holonyak '039 it was specified that the mostcritical areas of a semiconductor device to passivate were those inwhich the majority of the light generated by the light emitting diode(LED) are transmitted. This is based on the belief that corrosion wasaccelerated by photon interactions. Holonyak '039 also specifies theneed to control the temperature and time of the oxide growth period sothe thickness of the native oxide growth is within a particularthickness range. Specifically, the native oxide film must be thickerthan 0.1 um to avoid pinholes or cracks in the film, but thinner than7.0 um which can cause cracks in the film due to internal stress. Thecracks can prevent complete passivation and result in light output lossduring WHTOL tests. In Holonyak '039 it also stated that that the watervapor oxidation temperature range should be 375° C. to 550° C.,preferably from 450° C. to 550° C., and the most preferable oxidationtime is 0.25 hour to 2 hours.

In light emitting devices it is often desirable to incorporate wide bandgap, high Al content layers for improved carrier confinement, carrierinjection, wave guiding-properties, etc. For example, it is known thatthe emission efficiencies of red-emitting aluminum gallium arsenide(AlGaAs) LEDs can be improved by increasing the Al-mole fraction, x, ofthe high-composition Al_(x) Ga.sub.(1-x) As confining layers immediatelyadjoining the active layer. However, the destructive oxide degradationproblems have limited the content of these Al_(x) Ga.sub.(1-x) As layersto the range where the Al mole-fraction, x, is less than 0.75. The molefraction, x, indicates the amount of Al in the layer and is defined asthe fractional composition of Al to the Group III element in the layer.

The prior art has shown the performance of Al-bearing semiconductordevices can be greatly improved through the use of native oxidepassivation. Although many issues have been addressed, there is still noviable method for using this technique in high volume manufacturing. Tosuccessfully implement this technology, it is critical to haveprocessing techniques that can be used to passivate the device areasthat have the greatest impact on the device performance and reliability.

DISCLOSURE OF THE INVENTION

It has been discovered that the most critical areas for passivation arethe highest aluminum (Al)-content exposed layers of the device. ForAl-bearing substrate AlGaAs LEDs, the confining layers adjoining theactive layer possess the highest Al content. Failure analysis ofnon-passivated WHTOL-aged Al-bearing substrate AlGaAs LEDs indicatesthat corrosion occurs the fastest at the exposed surfaces of the high-Alcontent confining layers. By placing a high-quality native oxide at theexposed surfaces of the high-composition Al-content confining layerswhich protect from the formation of the `poor` oxide, it is possible forLEDs to retain essentially their same light output after 2,000 hours ofWHTOL testing.

Although passivating the highest exposed Al-content layers improves theWHTOL degradation, partial passivation results in only partial WHTOLprotection. It is desirable and optimal to passivate all or the majorityof the exposed Al-bearing layers in the light-emitting device structure.Such full passivation results in the optimal WHTOL performance forAl-bearing substrate AlGaAs LEDs. However, a consequence of full-waferprocessing is that it is very difficult (or nearly impossible) tocompletely expose all edges of the devices (by singulating them) priorto oxidation. The present invention provides a method of exposing themajority of the edges, especially those with the highest Al-bearinglayers, leaving the remaining layers intact and connected. Thisstructure can be realized by etching mesas (using wet chemical and/ordry plasma processes), sawing partially through the wafer, or acombination of both. Such processing facilitates the exposure of thelayers most prone to degradation for oxidation-passivation whileallowing full-wafer processing. The oxidation can occur prior to orafter the deposition of the top and/or bottom metallization layers orcontacts. Areas that are left connected should be of the lowestAl-content (or be Al-free). If the connected layers do contain Al, thesurface areas of these layers should be kept to a minimum compared tothe remainder of the device area. In addition, it is preferable that thenon-oxidized exposed surfaces of the connected layers pass a minimumamount of light after singulation under device operation to minimize anyphoton-assisted degradation.

In an Al-free substrate AlGaAs LED, a thick GaAs substrate can beemployed as a connected carrier layer, allowing all of the Al-bearinglayers to be exposed to oxidation-passivation. Again, depending on theoxidation conditions, the lower Al-content active layer may or may notbe passivated.

For Al-free and Al-bearing substrate AlGaAs LEDs, it is necessary tooxidize the LEDs from 500 to 625° C. for 1 to 60 minutes to ensurepassivation. It has been determined that 600° C. for 5 minutes gives thebest results for standard Al-bearing substrate AlGaAs LEDs and 550° C.for 6 minutes is optimal for Al-free substrate AlGaAs devices. DifferentAl contents need different temperatures to create a robust enough oxidefor moisture resistance. The thickness of the oxide can also play apart. If an oxide is too thin, it can have pinholes, but if it is toothick, in can stress the device and not let as much light out. Also, themetal contacts can degrade when exposed to high temperatures for a longtime, causing turn-on voltage issues.

The ability to successfully passivate high Al-content composition layersfrom WHTOL degradation allows the present invention to incorporate wideband gap, high Al-content composition layers for improved carrierconfinement, carrier injection, wave guiding properties, etc. Theemission efficiencies of the AlGaAs LEDs can be improved by increasingthe Al-mole fraction of the high-composition confining layersimmediately adjoining the active layer. Native oxidation passivationallows the increase in the Al mole fraction beyond traditional values.

Further features and advantages of the present invention will becomeapparent to those skilled in the art from a reading of the followingdetailed description when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of an Al-bearing substrate AlGaAswafer and one device after wafer singulation;

FIG. 2 is a Scanning Electron Micrograph (SEM) cross-section of aAl-bearing substrate AlGaAs LED that has undergone 2000 hours of WHTOLstressing, displaying a poor, optically absorbing oxide on the exposed,high Al-containing layers;

FIG. 3 is a WHTOL LOP degradation curve as a function of stress time fornon-passivated and passivated (600° C. for 5 min.) Al-bearing substrateAlGaAs LEDs;

FIG. 4 is a SEM cross-section of a passivated LED (600° C. at 5 min.)that has undergone 2000 hours of WHTOL stress, displaying a protectivenative oxide growth on the high-Al confining layers;

FIG. 5 is a schematic cross-section of portion of a semiconductor wafercontaining Al-bearing substrate AlGaAs device structures with a mesapartially separating the devices;

FIG. 6 is a WHTOL LOP degradation curves comparing die-level oxidizedand wafer-level oxidized Al-bearing substrate AlGaAs LEDs;

FIG. 7 is a schematic cross-section of a semiconductor wafer containingAl-free substrate AlGaAs device structures with a mesa partiallyseparating the devices;

FIG. 8 WHTOL LOP degradation curves for Al-bearing substrate AlGaAs LEDspassivated at different time/temperature conditions; and

FIG. 9 WHTOL LOP degradation curves for Al-free substrate AlGaAs LEDspassivated at different time /temperature conditions.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1 (PRIOR ART), therein is shown a semiconductorwafer 10 which includes two semiconductor devices such as LEDs 12 and12'. The LEDs 12 and 12' are separated into individual LEDs by a priorart singulation technique of being scribed on the surface at thelocation indicated by the arrow 14 and broken, leaving the faces 16 aspart of the LEDs 12 and 12'. The LED 12 has a carrier layer that is ap-type Al-bearing substrate 22, which may be a transparent substrate.Above the Al-bearing substrate 22 is a p-doped confining layer 30,p-doped active layer 32, and an n-doped confining layer 34. These layersform the double heterostructure, light-emitting device. The confininglayer 34 is topped by an n-doped contact layer 36. Above the n-dopedcontact layer 36 is a metallization layer 38 and, below the Al-bearingsubstrate 22, is another metallization layer 40.

The LEDs 12 and 12' may be Al-bearing substrate AlGaAs devices with thefollowing structure from top to bottom.

    ______________________________________                                                        possible thickness,                                                                             possible                                      Al.sub.x Ga.sub.1-x As range, x t range, t                                  ______________________________________                                        n-contact                                                                              x = 0.6    0-0.40   15 μm                                                                             0.01 μm-50 μm                         layer 36                                                                      n-confining x = 0.75 0.5-0.75 9 μm 0.1 μm-50 μm                      layer 34                                                                      p-active x = 0.39 0-0.40 2 μm 0.05 μm-5 μm                           layer 32                                                                      p-confining x = 0.75 0.5-0.75 3.5 μm 0.1 μm-50 μm                    layer 30                                                                      p-Al-bearing x = 0.75-0.60 0.5-0.75 56 μm 50 μm-500 μm                                                  substrate 22                             ______________________________________                                    

The Al mole-fractions and epitaxial layer thickness used in the presentinvention are specified. Typical ranges of the aluminum mole faction forthe different layers are also specified (mole fractions can have eithera fixed value or can be graded). It should be understood that the molefractions could be different in each of the layers.

The mole fraction of the active layer 32 determines the wavelength ofthe emitted light, with x=0.40 emitting red light and x=0 emittinginfrared light. Typical die sizes can vary from a 5 mil×5 mil diefootprint to a 100 mil×100 mil die footprint. Typical AlGaAs die sizesare 10 mil to 12 mil.

Referring to FIG. 2, therein is shown a Scanning Electron Micrograph(SEM) cross-section of an Al-bearing substrate AlGaAs LED that has beensubjected to 2000 hours of WHTOL stressing. The poor oxide 31 and 33appear on the layers above and below the active layer 32. An analysis ofthis oxide-induced failure mode led to the discovery that it possesses achemical dependency on the concentration of Al in the AlGaAs layers 22,30, 32, and 34. It was further discovered that the corrosion occurs thefastest at the exposed surfaces of the high Al confining layers 30 and34 that have the highest Al mole-fraction. This result was unexpected inview of Holonyak '039 since the photon flux through the confining layers30 and 34 is not significantly more than through the other layers.

Referring now to FIG. 3, therein is shown one of a plurality of WHTOLLOP (Light Output Power) degradation curves as a function of stresstime. Passivated (curve 35) and non-passivated (curve 37) Al-bearingsubstrate AlGaAs LEDs were used to assess the passivation process asapplied to the Al-bearing substrate AlGaAs die of FIG. 1 (PRIOR ART). Awide variety of oxidation times and temperatures were examined and itwas discovered that 600° C. for 5 min. shown by curve 35 in FIG. 3 gavethe best passivation in terms of WHTOL device performance. The LOP ofthe non-passivated control LED was degraded by 85% after 2000 hours ofWHTOL stressing. The oxidized sample was a fully-passivated sample,showing essentially no degradation after an initial LOP spike at 24hours which may be due to interface states, dopant redistribution orother factors.

Referring now to FIG. 4, a SEM cross-section of a passivated LED shows athin (˜1 um), high-quality oxide layer 39 is present that was able toprevent the poor oxide growth shown in the non-passivated LED of FIG. 2.

Referring now to FIG. 5, therein is shown a semiconductor wafer 41,which includes two semiconductor devices such as LEDs 43 and 43'. Thesame layers as in FIG. 1 (PRIOR ART) are given the same numbers. TheLEDs 41 and 41' are etched into mesas 43 and 43', passivated by thenative oxide water vapor oxidation, and split, or singulated intoindividual LEDs by sawing or scribing and breaking. The singulatingwould leave the faces 45 and 45', respectively, as part of LEDs 43 and43'. Also shown in FIG. 5 are the areas of native oxide passivation 46,48, and 46A where the oxide thickness can range from 0.1 to 7.0 um.

While the best approach to passivation would involve growing a nativeoxide on all the exposed Al-containing surfaces, this may not bepossible for manufacturing reasons since this would require additionalhandling of the individual devices after singulation. For practicalpurposes, it may not be possible to passivate all Al-bearing surfaces ofthe die. Generally, for manufacturing and cost, it is preferable toperform the wafer-fabrication processes (e.g., the high-temperaturewater-vapor oxide passivation of the Al bearing layers) in full-waferform to minimize handling, cost, and throughput issues associated withindividual devices. This is especially important in the manufacture LEDswhere the volumes are typically in the millions to hundreds of millionsof devices fabricated per month. However, a consequences of full-waferprocessing is that it is very difficult (or nearly impossible) tocompletely expose all surfaces of the devices (by singulating them)prior to oxidation.

The structure of FIG. 5 provides a solution which is to expose themajority of the surfaces, especially those with the highest Al-bearinglayers, leaving the remaining layers intact and connected. In thepractice of the present invention, LEDs 43 and 43' are made from thesame Al-bearing substrate AlGaAs LED material described above with thesame layer structure. Although the substrate 22 is Al-bearing, mesaetching is used to expose the highest-Al containing layers most prone todegradation, specifically the two confining layers 30 and 34 withAl-mole fraction of 0.75. The wafer is still interconnected via theconnected portion 23 of the Al-bearing substrate layer 22. The wafer isoxidized at 600° C. for 5 min. after which the wafer is singulated intoindividual die via a scribe and break technique. The mesa-etched devicesthat were oxidized in wafer-form result in partial passivation. The mesaetching exposes the top Al-bearing layers 30, 32, 34, and 36 of FIG. 5for native oxide passivation. The faces 45 and 45' are exposed onlyafter the oxidation step, and do not have a protective native oxide.

Referring now to FIG. 6, therein is shown a comparison of the WHTOL LOPdegradation curves for the die-level passivated device (curve 50), amesa-etched passivated device (curve 52) of FIG. 5, and non-passivatedcontrol samples (curves 54 and 56). The die, which is oxidized indie-form, results in full-passivation. The control samples display asevere 85% LOP degradation after 2000 hours. The wafer level oxidizeddie display partial passivation with 40% LOP degradation. SEMcross-sections confirm the top half of the die shows no corrosion, whilethe bottom half displays a poor, optically-absorbing oxide on theAl-bearing substrate layer 22 after 2000 hours of stress.

As design considerations, it should be noted that oxidation can beperformed prior to or after deposition of the metallization layers 38,40, 42, and 44. Ideally, the Al-bearing substrate 22 should be of thelowest Al-content composition for good passivation, although this maycompromise the LOP. For practical purposes to facilitate handling in themanufacture of III-V compound semiconductor devices, the connectedportion 23 of the Al-bearing substrate 22 should preferably be from 2 to6 mils in thickness after forming mesas. For an Al-bearing substrate,the thickness should be minimized to minimize exposed Al. In addition itis preferable that the exposed surfaces of the connected layers aftersingulation pass a minimum amount of light under device operation tominimize any photon-assisted degradation which may occur as described inHolonyak '039. Also, depending upon the oxidation conditions, the lowerAl-containing active layer may or may not be oxide passivated. Forexample, lower oxidation temperatures than those required to passivatethe active layer may be required to minimize any dopant diffusion or topreserve the integrity (electrical and/or mechanical) of any previouslyapplied metallizations. Another technique is to oxidize the devicefirst, mask and etch the oxide, then put down the metal contacts. It mayalso be preferable to design die LED 12 so that the non-passivated areais p-type rather than n-type since n-type layers have been observed todegrade faster than p-type layers. Although not ideal, a wafer leveloxidized device will exhibit significantly improved performance comparedto a device with no oxidation.

Referring now to FIG. 7, therein is shown a semiconductor wafer 110which includes two semiconductor devices such as LEDs 112 and 112'. TheLEDs are formed into mesas 114 and 114' that will eventually be split,or singulated, into individual LEDs by sawing or scribing and breaking.The singulation would leave the faces 116 and 116' respectively, as partof the LEDs 112 and 112'. The LED has a carrier layer that is an n-typeAl-free substrate 122 which can be either absorbing or transparentsubstrate. Above the Al-free substrate 122 is an n-doped buffer layer126. Above the buffer layer 126 is an n-doped confining layer 130, ap-doped active layer 132, and a p-doped confining layer 134. The lastthree layers form the double heterostructure, light-emitting device.Above the confining layer 134 is the p-doped buffer layer 135 and then ap-doped contact layer 136. Above the p-doped contact layer 136 is ametallization layer 138 and, below the Al-free substrate 122 ismetallization layer 140.

The LED 112 may be an Al-free substrate AlGaAs device with the followingstructure from top to bottom:

    ______________________________________                                                        possible thickness,                                                                             possible                                      Al.sub.x Ga.sub.1-x As range, x t range, t                                  ______________________________________                                        p-contact                                                                              x = 0.27   0-0.40   1 μm                                                                              0.01 μm-5 μm                          layer 136                                                                     p-buffer x = 0.50 0.5-0.75 16 μm 0.1 μm-50 μm                        layer 135                                                                     p-confining x = 0.75 0.5-0.75 10 μm 0.1 μm-50 μm                     layer 134                                                                     p-active x = 0.39 0-0.40 1.5 μm 0.05 μm-5 μm                         layer 132                                                                     n-confining x = 0.75 0.5-0.75 6 μm 0.1 μm-50 μm                      layer 130                                                                     n-buffer x = 0.60 0.5-0.75 5 μm 0.1 μm-50 μm                         layer 126                                                                     n-Al-free x = 0 0 250 μm 50 μm-500 μm                                substrate 122                                                               ______________________________________                                    

The Al mole-fractions and epitaxial layer thickness used in the presentinvention are specified. Typical ranges of the aluminum mole faction forthe different layers are also specified (mole fractions can have eithera fixed value or can be graded). It should be understood that the molefractions could be different in each of the layers.

For the AS AlGaAs LED 112, all Al-containing layer are passivated priorto singulation since the Al-free GaAs substrate 122, can be employed asthe connected carrier layer. This allows all of the Al-containing layersto be exposed to oxide passivation. It should be noted that a lowercomposition Al-containing layer (0<x<0.3) may be employed at the contactsurfaces to assist in the formation of low resistance contacts. Again,depending on the oxidation conditions, the lower Al-containing active orcontact layer may or may not be oxidized. Also, the need to oxidizethese surfaces is less important since the lower compositionAl-containing compound degrade at significantly lower rates. Typical diesizes can vary from a 5 mil×5 mil die footprint to a 100 mil×100 mil diefootprint.

Holonyak '039 indicates that the water vapor oxidation temperature rangeshould be 375° C. to 550° C., preferably from 450° C. to 550° C., andthe most preferable time period is 15 minutes to 2 hours. However, forAl-free and Al-bearing substrate AlGaAs LEDs it has been discovered thatit is necessary to oxidize the samples from 500° C. to 650° C. for oneminute to sixty minutes but generally less than 10 min. to ensure thatthe samples are well passivated and device performance is not degraded.

Referring now to FIG. 8, therein is shown the WHTOL LOP degradationcurves for Al-bearing substrate AlGaAs LEDs passivated at different timeand temperature conditions (curves 60, 62, 64, and 66). Optimalpassivation seems to occur at approximately 600° C. for 5 minutes (asshown by curve 60). The thickness of the oxide alone (typically between0.1 um and 7.0 um) will not guarantee good passivation. This isespecially evident in FIG. 8 because the thicker 1.6 μm oxide depositedat 500° C. for 20 minutes (curve 62) does not passivate as well as thethinner oxide deposited at 600° C. for 5 minutes (curve 60).

Referring now to FIG. 9, therein is shown the WHTOL LOP degradationcurves for Al-free substrate AlGaAs LEDs passivated at different timeand temperature conditions (curves 70, 72, 74, and 76). In this case,optimal passivation seems to occur at approximately 550° C. for 6minutes (as shown by curve 70). Device structures created underdifferent growth conditions may need different temperatures to create arobust enough oxide for moisture resistance.

The ability to successfully passivate high-Al composition layers fromWHTOL degradation offers the opportunity to incorporate new devicedesigns that were previously unfeasible. For example, in light-emittingdevices it is often desirable to incorporate wide band gap, high-Alcomposition layers for improved carrier confinement, carrier injection,wave guiding properties, etc. For example, it is known that the emissionefficiencies of red-emitting AlGaAs LEDs can be improved by increasingthe Al mole fraction of the high-composition confining layersimmediately adjoining the active layer. However, destructive oxidedegradation problems have limited the composition of these layers tox˜0.6 to 0.75 in non-passivated devices. Using the passivation schemesdescribed herein, it should be feasible to increase the composition ofthese confining layers to x˜0.75 to 1.0, maximizing both LOP and WHTOLperformance.

While the invention has been described in conjunction with a specificbest mode, it is to be understood that many alternatives, modifications,and variations will be apparent to those skilled in the art in light ofthe foregoing description. Accordingly, it is intended to embrace allsuch alternatives, modifications, and variations which fall within thespirit and scope of the included claims. For instance, differentmaterial systems may include InAlGaAs, AlGaAsP, InAlGaAsP, InAIGaN, andcombinations thereof. Given these combinations of materials withconfining layers and an active layer in between, only two of the threelayers have to have mole fractions of aluminum and only the layercontaining the highest mole fraction of aluminum needs to be passivated.Also, this invention is applicable to other light-emitting devices; i.e.vertical cavity surface emitting lasers. All matters set forth herein orshown in the accompanying drawings are to be interpreted in anillustrative and non-limiting sense.

The invention claimed is:
 1. A method of manufacturing Groups III-Vlight emitting semiconductors, the semiconductors having an active layerdisposed between two layers with two of the layers having predeterminedmole fractions of aluminum, processed on a semiconductor wafer to thepoint of passivation, comprising the steps of:forming mesas on thesemiconductor wafer to expose surfaces of the two layers havingpredetermined mole fractions of aluminum in the Groups III-Vsemiconductor layers therein; passivating the exposed surface of thelayer containing the highest predetermined mole fraction of aluminum ofthe two layers having predetermined mole fractions of to form a nativeoxide using a water vapor oxidation process; and singulating said mesasto form individual light emitting semiconductors.
 2. The method asclaimed in claim 1 wherein said step of:passivating the exposed surfaceof the layer containing the highest predetermined mole fraction ofaluminum of said two layers having predetermined mole fractions ofaluminum to form a native oxide is performed from 500° C. to 650° C. 3.The method as claimed in claim 1 wherein said step of:passivating theexposed surface of the layer containing the highest predetermined molefraction of aluminum of said two layers having predetermined molefractions of aluminum to form a native oxide is performed from 500° C.to 650° C. for one minute to sixty minutes.
 4. The method as claimed inclaim 1 including a step of:forming the layers from a compound selectedfrom a group consisting of AlGaAs, InAlGaAs, AlGaAsP, InAlGaAsP,InAIGaN, and combinations thereof.
 5. The method as claimed in claim 1wherein said step of:forming said mesas on the semiconductor waferexposes the surfaces of the two layers having predetermined molefractions of aluminum in the Groups III-V semiconductor layers to athickness of from two to six mils.